This invention relates generally to blast furnaces and to the control of the furnace top pressure thereof. More particularly, the invention relates to a system for changing over the control of the furnace top pressure in a blast furnace system including a septum valve capable of controlling the top pressure and a blast furnace gas energy recovery turbine installed in a manner to bypass the septum valve and being capable also of controlling the top pressure, that is, a blast furnace gas energy recovery turbine having a so-called front pressure control capability. Elsewhere throughout the present specification, a blast furnace gas energy recovery turbine as mentioned above is referred to simply as a "gas turbine."
In general, the exhaust gas generated in a blast furnace for iron making is passed through a septum valve and supplied to equipment such as a boiler, and recovery of energy retained by the exhaust gas is carried out. The ordinary practice in this case in adjusting the furnace top pressure is to hold it at a preset pressure value by automatically regulating the degree of opening of the above mentioned septum valve.
Alternatively, as a means for minimizing the throttling loss of the exhaust gas energy due to the septum valve, there is employed a system wherein a pipe line bypassing the septum valve is provided, and a gas turbine is installed in this pipe line to receive the total gas quantity, exclusive of leakage gas, at the time of full closure of the spectrum valve. The gas turbine thereby causes this quantity of the gas to expand and release its retained energy to drive a generator coupled to the gas turbine. At the same time, by suitably adjusting a speed regulating valve installed as an accessory of the gas turbine, the furnace top pressure is automatically controlled to a preset pressure.
However, the above described various means of the known systems are respectively practicable as independent control means, but when at attempt is made to use them in combination, it is found that there is no interrelationship therebetween. On the one hand, in the practical operation of the blast furnace system, there are cases where the gas turbine drive is necessary and cases where it is not, because of the fluctuations of the generated exhaust gas quantity and other reasons. Moreover, the variation of this situation is not regularly periodical but, rather, is irregular, occurring suddenly at times.
In order to cope with this fluctuation and thereby to utilize the generated gas energy in an effective and practical manner, a changeover operation for changing over the motive power source for furnace top pressure control each time from the septum valve to the gas turbine and, conversely, from the gas turbine to the septum valve becomes necessary. This operation, however, is extremely complicated, and its reduction to practice is very difficult. Accordingly, it has heretofore been almost impossible to expect smooth changeover of top pressure control in a blast furnace system of the type here being considered.